THRESHOLD CONDITIONS FOR IODINE-INDUCED STRESS-CORROSION CRACKING OF UNIRRADIATED ZIRCALOY-4 TUBING UNDER INTERNAL PRESSURIZATION

The results are presented of stress corrosion cracking (SCC tests in which nuclear power reactor grade zircaloy-4 tubing specimens were internally pressurized with a mixture of helium and iodine at (633 ± 5) K. Both as-received and artificially preflawed specimens were tested at an initial iodine availability of ~60 g/m2 zircaloy surface. It is shown that the failure times in these tests correlate more reliably with hoop stress than with nominal stress intensity or failure strain, and that a threshold hoop stress of ~295 MPa exists for SCC failure within test times up to 605 ks. The origin of this threshold stress is discussed and it is concluded that the observed behavior is consistent with either a critical stress or a critical strain rate being required for the formation of iodine-induced stress corrosion cracks in unirradiated zircaloy tubing. © 1979.